Remove all the project code.
Ok, "remove all the existing project code", obviously there is then new code based on my suggestion to be added.
You'll need to explain in a *lot* more detail what you have in mind.
Then make "projects" to use the equivalent of geany -c but selected from the GUI. That way there is no duplicate code and all settings can be setup for "projects" including tweaked filetypes.
That seems very... complicated and unweildy to use and implement.
Removing *all* or even just *existing* project code is essentially the removal of all/existing project functionality. That would mean, no more `.geany` project files. You appear to want to replace it with multiple copies of... the entire geany config folder? (Tweaking filetypes involves files other than just `geany.conf`.)
...
You don't appear to really mean "remove all [/existing] project code", but to modify/extend projects in some undefined manner.
And to add, changes to stash would be instrumental to this changeover.
What changes? How do you expect it will be used?